1. Field of the Invention
The present invention relates to a method for driving a plasma display panel (PDP).
As a display device for a television set with a large screen, a surface discharge type AC plasma display panel is commercialized. This surface discharge type has first and second display electrodes that are arranged in parallel on a front side or a backside substrate as anodes and cathodes of display discharge for securing intensity. In the surface discharge type, three kinds of fluorescent material for color display, which is red, green and blue fluorescent material, can be disposed separately from the pair of display electrodes in the direction of the thickness of the panel. Thus, a deterioration of the fluorescent layer due to an ion impact upon discharge is reduced so that a long life color screen can be realized.
If the screen becomes larger, it is more difficult to make a cell structure uniform. If the cell becomes smaller, a small difference of the cell structure affects the discharge characteristics more largely. Therefore, in order to promote a wide screen and a high definition of the screen, a driving method is necessary that can permit a variation of the discharge characteristics and has a large margin of voltage.
2. Description of the Prior Art
As an electrode matrix structure of the surface discharge type plasma display panel, a "three-electrode structure" is known widely, in which an address electrode is arranged to cross a pair of display electrodes. The three-electrode structure basically has a pair of display electrodes for each row. An arrangement distance of the display electrodes in each row (a surface discharge gap length) is set to several dozens of microns so that the discharge can be generated by application of a voltage at approximately 150-200 volts. An electrode gap between neighboring rows is set to a value that is sufficiently larger than (several times of) the surface discharge gap length. The arrangement distance of the display electrode in each row is different from that between the rows. In another three-electrode structure, display electrodes whose number is one larger than the number n of the screen rows are arranged at an equal pitch, and the surface discharge is generated by neighboring electrodes as an electrode pair.
The display utilizes a memory function of a dielectric layer that covers display electrodes. Namely, addressing is performed for forming a charged state corresponding to a display contents in the line scanning format, and then a sustaining voltage Vs having alternating polarity is applied to the display electrode pair of each row. One of the display electrodes (a second display electrode) is used as a scanning electrode for addressing, and the address electrode is used as a data electrode.
The sustaining voltage Vs satisfies the following equation (1). EQU Vf-Vw&lt;Vs&lt;Vf (1)
Here, Vf is a discharge starting voltage and Vw is a wall voltage between display electrodes.
When the sustaining voltage Vs is applied, a cell voltage Vc (a sum of the applied voltage and the wall voltage, which is also referred to as an effective voltage Veff) exceeds the discharge starting voltage Vf in the cell having the wall charge, so that the surface discharge is generated along the surface of the substrate. By shortening the application period of the sustaining voltage Vs, an apparent continuous lighting state is obtained.
Since the cell of the plasma display panel is a binary light emitting element, middle tones are reproduced by setting the number of discharge times per one field in each cell in accordance with a gradation level. Color display is one of the gradation displays, and the display color of the display depends on a combination of intensity of three fundamental colors. The word "field" means a unit image of a sequential image display in this specification. In the television, it means each field of an interlace format frame, while in a non-interlace format such as a computer output, it means a frame itself. For the gradation display, one field includes plural subfields having weights of intensity, and the total number of discharge of one field is set by combining on and off of each subfield. If the application period (drive frequency) of the sustaining voltage Vs is constant, the application time of the sustaining voltage Vs is different between different weights of intensities.
In general, an addressing preparation period is assigned to the subfield along with an addressing period and a sustaining period. At the end of the sustaining period, cells with remaining wall charge and cells without remaining wall charge are mixed. Therefore, the charged states of all cells are uniformed in the addressing preparation period so that the reliability of the addressing is improved. Fundamentally, all cells are set to non-charged state in the addressing preparation period for a writing format addressing, while a constant quantity of wall charge is formed in all cells for an erasing format addressing. However, there is a little variation of discharge characteristics between cells in fact. Therefore, if the charge quantity of all cells is made uniform, the voltage margin of addressing is narrowed by the variation of the characteristics.
A method of performing a preparation process is proposed in U.S. Pat. No. 5,745,086 and Japanese unexamined patent publication No. 10-157107. The method includes a charge forming step and a charge adjusting step for enlarging the voltage margin of the addressing. In the charge forming stage, wall voltage having the same polarity is generated in all cells. It is not required to control the charge quantity strictly. In the charge adjusting step, a slowly increasing voltage having a small gradient (a ramp voltage used here) is applied so as to decrease the wall voltage to an appropriate value.
The principle of the charge adjusting will be explained as follows. When applying an appropriate mild ramp voltage as the conventional driving method shown in the Japanese unexamined patent publication No. 10-157107, the cell voltage Vc reaches the discharge starting voltage Vf, and after that a weak discharge occurs periodically so that the wall voltage drops gradually. The cell voltage alters a little with the drop of the wall voltage and the increase of the application voltage. However, it is kept substantially at the discharge starting voltage Vf. In addition, if an extremely gentle ramp voltage is applied as the conventional method shown in the U.S. Pat. No. 5,745,086, the cell voltage Vc is close to the discharge starting voltage Vf and does not exceed the same while a continuous current flows so that the wall voltage drops gradually. In this specification, the discharge for decreasing the wall voltage gradually is referred to as a "charge adjusting discharge," which includes a state of generating a periodical minute discharge, a mixing state of discrete discharge and continuous discharge, and a state of continuous discharge. When the application of the ramp voltage is finished, the cell voltage Vc drops to the value Vwr of the wall voltage at the end of the charge adjusting discharge. This value Vwr corresponds to the difference between the discharge starting voltage Vf and the maximum value Vr of the applied ramp voltage as shown in the equation (2). EQU Vwr=Vf-Vr (2)
It is obvious from the equation (2) that the value Vwr of the wall voltage does not depend on the value of the wall voltage at the start of the application of the ramp voltage, but depends on the setting of the maximum value Vr of the applied voltage. Therefore, in the charge forming stage, a wall voltage is generated in the range that can generate the charge adjusting discharge after that.
In the addressing after the above-mentioned charge adjusting, a pulse voltage that has the same polarity as the ramp voltage applied in the charge adjusting step is applied for generating an address discharge. Using the peak value (amplitude) Vp of the pulse voltage, the cell voltage Vc when applying the pulse voltage is expressed in the equation (3), i.e., it is the discharge starting voltage Vf plus .DELTA.V (=Vp-Vr). If the .DELTA.V is positive, the discharge occurs. If the .DELTA.V is negative, the discharge does not occur. EQU Vc=Vwr+Vp=Vf-Vr+Vp=Vf+.DELTA.V (3)
Here, .DELTA.V is Vp-Vr
The values of Vr and Vp are set properly so that the discharge occurs. Thus, the differential voltage .DELTA.V between the cell voltage Vc and the discharge starting voltage Vf becomes uniform even if the discharge starting voltage Vf has a variation among cells, so that the intensity of the discharge becomes uniform in all cells. Thus, the voltage margin is enlarged.
The above-mentioned U.S. patent and the Japanese unexamined patent publication No. 10-157107 disclose the driving method, in which a ramp voltage is applied simultaneously to two pairs of electrodes, one pair is the scanning electrode for selecting cells of addressing and the address electrode (this is referred to as an interelectrode YA), and the other pair is the display electrodes for sustaining (this is referred to as an interelectrode XY), and then a ramp voltage is applied simultaneously again for charge adjusting. Namely, the preparing process in the conventional method and the prior art includes a first step for generating a charge forming discharge at the interelectrode YA and the interelectrode XY, and a second step for generating a charge adjusting discharge at the interelectrode YA and the interelectrode XY. An increasing voltage is used for the charge forming discharge, so that the discharge intensity can be suppressed to the minimum and undesired light emission can be avoided.
In the experimental process researching the optimal application condition for applying the conventional driving method of the prior art (the driving method of performing two-step preparation), it was discovered that there is a substantial difference of the discharge characteristics of the address discharge between the "previously lighted cell" and the "previously unlighted cell". If this difference becomes small, the voltage margin increases. The previously lighted cell means the cell that was lighted in the last sustaining operation performed before the present addressing, and the previously unlighted cell means the cell except the previously lighted cell.
FIG. 21 shows voltage waveforms of the driving method of performing the two-step preparation process. FIG. 22 is a graph showing the dependence of the address discharge on the voltage in the driving method of performing the two-step preparation process. FIGS. 23A and 23B show wall voltage at the interelectrode XA in the driving method of performing the two-step preparation process.
The amplitude of the voltage pulse applied to the display electrodes X, Y and the address electrode A (a bias potential with respect to the GND) is selected as shown in Table 1 for measuring the integral value of the light emission during the display period. The display pattern includes three patterns of red color, green color and blue color, each of which is divided into the case where the cell to be lighted is the previously lighted cell and the case where the cell to be lighted is the previously unlighted cell. Thus, for total six kinds of patterns, the state of the addressing was studied using a parameter of the address voltage Va. The axis of ordinates in FIG. 22 has a relative scale standardized using the integral value of the light emission as one when all cells to be lighted are lighted properly in the display period.
TABLE 1 Addressing Preparation First Step Second Step Addressing Display V1a V1x V1y V2x V2y Vy Vsc Va Vs 0 0 430 170 0 -20 60 * 170 (The unit is volts and * is a parameter)
As shown in FIG. 22, there is a substantial difference of the addressing characteristics between the case of the previously lighted cell and the case of the previously unlighted cell in the red cell and the green cell. The characteristics are different depending on the color because the charge characteristics of the fluorescent material and the shape (especially the thickness of the film) of the fluorescent layer are different.
In order to evaluate the charge adjusting by the driving method of performing the two-step preparation process, the wall voltage at the interelectrode XA at the end of the charge adjusting is measured for various display patterns. The interelectrode XA means between the first display electrode X that is not a scanning electrode and an address electrode A. A ramp voltage was applied instead of the addressing operation in the measurement, so that the light emission can be observed by an oscilloscope. When the sum of the increasing applied voltage and the wall voltage reaches the discharge starting voltage, discharge occurs to emit light. FIG. 22 shows the applied voltage and the transition of the output of the light emission sensor in the condition that the display pattern is all white and the voltage of the addressing preparation is selected in accordance with Table 2.
TABLE 2 V1a V1x V1y V2x V2y 0 0 440 170 0 (The unit is volts)
As shown in FIG. 23A, the discharge occurred when the applied voltage is 4 volts for the previously unlighted cell. As shown in FIG. 23B, the discharge occurred when the applied voltage is -26 volts for the previously lighted cell. It was discovered that there is the difference of 30 volts of the wall voltage at the interelectrode XA depending on the display pattern.